perf build: Add ability to build with a generated vmlinux.h

Commit a8874665 ("perf bpf skels: Stop using vmlinux.h generated
from BTF, use subset of used structs + CO-RE") made it so that
vmlinux.h was uncondtionally included from
tools/perf/util/vmlinux.h. This change reverts part of that change (so
that vmlinux.h is once again generated) and makes it so that the
vmlinux.h used at build time is selected from the VMLINUX_H
variable. By default the VMLINUX_H variable is set to the vmlinux.h
added in change a8874665, but if GEN_VMLINUX_H=1 is passed on the
build command line then the previous generation behavior kicks in.

The build with GEN_VMLINUX_H=1 currently fails with:

    util/bpf_skel/lock_contention.bpf.c:419:8: error: redefinition of 'rq'
    struct rq {};
           ^
    /tmp/perf/util/bpf_skel/.tmp/../vmlinux.h:45630:8: note: previous definition is here
    struct rq {
           ^
    1 error generated.
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20230623041405.4039475-2-irogers@google.com
[ Format the error message and add a comment for GEN_VMLINUX_H ]
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/Makefile.config

@@ -680,6 +680,10 @@ ifdef BUILD_BPF_SKEL
   CFLAGS += -DHAVE_BPF_SKEL
 endif
 
+ifndef GEN_VMLINUX_H
+  VMLINUX_H=$(src-perf)/util/bpf_skel/vmlinux/vmlinux.h
+endif
+
 dwarf-post-unwind := 1
 dwarf-post-unwind-text := BUG
 

tools/perf/Makefile.perf

@@ -132,6 +132,8 @@ include ../scripts/utilities.mak
 # Define EXTRA_TESTS to enable building extra tests useful mainly to perf
 # developers, such as:
 #	x86 instruction decoder - new instructions test
+#
+# Define GEN_VMLINUX_H to generate vmlinux.h from the BTF.
 
 # As per kernel Makefile, avoid funny character set dependencies
 unexport LC_ALL
@@ -1084,7 +1086,21 @@ $(BPFTOOL): | $(SKEL_TMP_OUT)
 	$(Q)CFLAGS= $(MAKE) -C ../bpf/bpftool \
 		OUTPUT=$(SKEL_TMP_OUT)/ bootstrap
 
-$(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) | $(SKEL_TMP_OUT)
+VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)				\
+		     $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)	\
+		     ../../vmlinux					\
+		     /sys/kernel/btf/vmlinux				\
+		     /boot/vmlinux-$(shell uname -r)
+VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
+
+$(SKEL_OUT)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
+ifeq ($(VMLINUX_H),)
+	$(QUIET_GEN)$(BPFTOOL) btf dump file $< format c > $@
+else
+	$(Q)cp "$(VMLINUX_H)" $@
+endif
+
+$(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
 	$(QUIET_CLANG)$(CLANG) -g -O2 -target bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
 	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
 

tools/perf/util/bpf_skel/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 .tmp
 *.skel.h
+vmlinux.h